On Tue, Oct 12, 2021 at 06:34:06AM -0000, Jeremiah Vivian wrote:
> So `ord` is already a really fast function with (last check before
> this thread was posted) 166 nsec per loop. But I'm wondering... doing
> `ord(a)` produces this bytecode:
>
> > 2 4 LOAD_NAME 1 (ord)
> > 6 LOAD_NAME 0 (a)
> > 8 CALL_FUNCTION 1
> > 10 POP_TOP
> > 12 LOAD_CONST 1 (None)
> > 14 RETURN_VALUE
>
> But doing `+a` only produces this:
>
> > 2 4 LOAD_NAME 0 (a)
> > 6 UNARY_POSITIVE
> > 8 POP_TOP
> > 10 LOAD_CONST 1 (None)
> > 12 RETURN_VALUE
Don't be fooled though, the UNARY_POSITIVE byte-code has to inspect the
argument `a` for a `__pos__` method, and if it exists, call it. So there
is a couple of hidden function calls in there.
But it is true that operators do save the cost of looking up the
function name.
> So an operator has its own bytecode, doesn't need to `LOAD_*` a
> function, and doesn't have the impact on performance when converting
> arguments to the format of the (after `POP()`ing every argument) TOS
> function and then calling that function.
If the lookup of a function is having a significant cost to you, perhaps
because you are calling the function in a really tight loop, there is
an optimization you can do. Suppose we have an enormous string with a
billion characters, and we run this:
a = 0
for c in enormous_string:
a += ord(c)
That looks up ord once for every character in the string. But if we move
the code to a function, and use a local variable, we can reduce those
lookups to one instead of a billion:
def func(enormous_string):
ordinal = ord
a = 0
for c in enormous_string:
a += ordinal(c)
return a
a = func(enormous_string)
That may give you a small percentage boost.
> And also, the unary `+` of
> strings only copies strings, which should be redundant in most cases.
Are you sure about that?
>>> s = +"a"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: bad operand type for unary +: 'str'
I don't think it works. What version of Python are you using?
--
Steve
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